Pulmonary arterial hypertension (PAH) is a disease characterized by the vascular remodeling of the pulmonary arterioles, including formation of plexiform and concentric lesions comprised of proliferative endothelial cells. We recently found that disruption of the G protein-coupled receptor signaling axis comprised of the ligand apelin and the receptor APJ leads to significant worsening of hypoxia induced pulmonary hypertension (PH) in mice, and found that the pathway is also perturbed in clinical PAH. To further characterize the downstream endothelial targets which may be utilized by apelin-APJ signaling to protect against the remodeling processes in PAH, we carried out a microRNA (miRNA) array analysis of pulmonary artery endothelial cells (PAECs) subjected to knockdown of apelin, APJ, or both. We identified two miRNAs (miR-424 and miR-503) that are highly expressed in PAECs, but are significantly downregulated in the context of apelin/APJ knockdown. Remarkably, these miRNAs are also markedly decreased in the experimental monocrotaline rat model of PH, as well as in PAECs isolated from patients with clinically diagnosed PAH. Our in vitro functional studies demonstrated that both miR-424 and miR-503 exert anti-proliferative, anti-angiogenic effects, via a mechanism that at least in part involves targeting of the FGF signaling pathway. We found that restoration of miR-424/503 expression in the monocrotaline induced model of PH by intranasal lentiviral delivery can significantly ameliorate the severity of the disease. Based o these preliminary data, we hypothesize that miR-424/503 play a critical role in maintaining pulmonary vascular homeostasis, and that disruption of their expression serves a critical contribution to the aberrant endothelial proliferation which is a hallmark of PAH. We propose the following aims to further evaluate the role of miR-424/503 in PAH: 1) Determine the transcriptional regulatory mechanisms of miR-424/503 in PAECs that involves targeting of the myocyte-specific enhancer factor 2 (MEF2) by apelin-APJ signaling, 2) Evaluate the efficacy of endothelial specific expression of miR-424/503 in experimental models of PH, and 3) Further validate the role for miR-424/503 in clinical PAH.